WASHINGTON -- Federal safety investigators on Thursday described damage to the battery from a Boeing 787 Dreamliner that caught fire in Boston as severe and said finding the cause of the problem won't happen overnight.

Deborah Hersman, chairman of the National Transportation Safety Board, said Thursday that the significance "cannot be overstated" of the fire Jan. 7 on a Japan Airlines plane and a smoldering battery on Jan. 16 that forced the emergency landing of an All Nippon Airways flight in Japan.

"The expectation in aviation is to never experience a fire aboard an aircraft," Hersman said.

Hersman said the fire damaged the plane's structure and its components within 20 inches of the battery called an auxiliary power unit (APU). Pictures showed a battery with eight adjacent cells packed in two rows of four cells charred black from the fire, as well as damage by the hot liquid, called electrolyte, inside the battery.

The lithium-ion battery from the Boston plane has signs of short circuits and a "thermal runaway," which is a chemical reaction that overheats the battery and cannot be controlled. But Hersman couldn't say yet which event came first or why they occurred.

“There is a lot of technical work and a lot of complex work to understand.”

Deborah Hersman, NTSB chairman

"It is answering the 'why' question that will ensure that the appropriate corrective actions are taken," Hersman said.

Besides Boeing, NTSB officials are working with: Securaplane Technologies in Tucson, which made the charger; United Technologies Aerospace Systems for a battery controller in Phoenix; and Kanto Aircraft instrument for battery monitoring in Japan. NTSB officials are also learning about lithium batteries from the Naval Surface Warfare Center's Carderock Division.

Investigators have X-rayed, CT-scanned and taken apart the Boston battery. So far, Hersman said, the damage reflects the symptoms of the problems rather than what caused them.

"We are looking for contaminants or defects," she said. "We are working very hard to determine what happened and why it happened."

Joseph Kolly, director of NTSB's office of research and engineering, says a thermal runaway could be caused by a short-circuit or damage from a neighboring cell that overheated or short-circuited.

"It just cascades down the row of neighboring cells," Kolly says.

Hersman couldn't say how long the investigation would take. But she said a draw-down test of the battery takes a week.

"This is not something that we are expecting will be solved overnight," Hersman said. "There is a lot of technical work and a lot of complex work to understand."

The NTSB update came the same day as an earnings call for United Airlines, which is the only U.S. carrier with the plane. United officials say after getting six Dreamliners last year, they are confident the problems will be solved, and they expect to get another two this year.

"History teaches us that all new aircraft have issues and the 787 is no different," says Jeff Smisek, United's chief executive officer. "We continue to have confidence in the aircraft and in Boeing's ability to fix the issues, just as they have done on every other new aircraft model they have produced."

Boeing has halted deliveries of the planes, with nearly 850 on order. Airlines have canceled flights into mid-February while waiting for a remedy.